The invention is concerned with scanning type superheterodyne receivers, and more particularly to scan-lock systems that utilize automatic frequency control of a local oscillator for receiving pulsed signals.
A scanning radar receiver is a typical example of such receivers. This type of receiver sweeps a selected bandwidth to search for a sequence of received radar pulses whose center frequency is not known precisely, but lies within the swept bandwidth. As the receiver is tuned across such a signal, it should stop its sweep and lock onto the received signal. After lock on, it must track any change in the received frequency which may be due from such sources as drift in the transmitted signal, changes in receiver components, or frequency changes due to doppler shifts.
Other types of systems using this type of receiver are satellite communication links using pulse code modulation, and missile command and control links. When receivers of this type are operated in the microwave region, little if any preselection or tuned circuits are used ahead of the local oscillator (LO). Therefore, the desired intermediate frequency produced by mixing of the local oscillator with an incoming frequency will be generated by a received signal either above or below the LO frequency. The receiver designer normally selects one of the relationships as the correct IF signal and regards the other as the image frequency.
Most receivers in the prior art have used analog circuitry for controlling the LO frequency. Commonly, an electronically tuned local oscillator is utilized as part of an automatic frequency control (AFC) loop in microwave receivers, but the design of proper AFC circuitry has presented certain problems. This circuitry must sweep the LO through its tuning range until a signal is encountered, and then after lock-on to a signal, it must use inputs from an IF operated discriminator in order to generate the tuning voltage for the LO, to maintain the LO in the locked condition.
Typically, for analog type circuitry, a sophisticated integrator is required that has the unfortunate characteristics of being slow to respond to input changes, that tends to drift with time, and requires elaborate sample and hold circuits to handle pulsed inputs. Additionally, and quite importantly, such analog circuits do not inherently provide protection against locking onto the unwanted image frequency.